Technical Field
[0001] The present invention relates to a honeycomb ceramics structure body which, for example,
can attain a high collection efficiency with a low pressure loss and can be suitably
used as a diesel particulate filter (DPF), and a method for producing the same.
Background Art
[0002] In these years, the diesel particulate filters (DPFs) for collecting particulates
discharged from diesel engines have been attracting attention, which are required
to attain high collection efficiency with low pressure loss.
[0003] As DPFs, the honeycomb structure bodies made of cordierite have been conventionally
used; in pursuit of such high collection efficiency with low pressure loss as described
above, the improvement has hitherto been made in the honeycomb structure bodies as
to the porosity and pore distribution thereof.
[0004] JP-A-9-77573 discloses a honeycomb structure having a specified the pore distribution
on the surface of the partition walls with an enlarged porosity and an enlarged average
pore size. JP-A-11-333293 describes honeycomb structure body having an enlarged porosity
in addition to thin partition walls of a prescribed value or less.
[0005] In addition, JP-B-7-38930 discloses the production method for a honeycomb structure
body having a high porosity by using a cordierite-forming raw material containing
a talc powder and a silica powder each composed of coarser particles of a prescribed
particle size or more. Japanese Patent No. 2726616 discloses a honeycomb structure
body having a specified pore distribution and surface roughness in addition to a high
porosity.
[0006] In the above described prior art, in order to increase the porosity, a cordierite-forming
raw material is pulverized into coarse particles, graphite, wood powder, and a foaming
agent are added as pore forming agents, or the like, but sufficiently satisfactory
effects have not yet been obtained.
[0007] More specifically, when a cordierite-forming raw material is pulverized into coarse
particles, the cordierite-forming reaction does not proceed to a sufficient extent,
so that it is difficult to attain a low thermal expansion. When graphite is used as
a pore forming agent, the following problems occur: the dielectric constant of a formed
body with addition of graphite is decreased, so that it becomes difficult to perform
a uniform drying of the formed body by the dielectric drying or the microwave drying
with increase in addition amount of graphite. Furthermore, the firing period at the
range from 800 to 1000°C is to be so elongated in the firing process that it is necessary
to suppress the rapid combustion of the graphite.
[0008] Moreover, when the starches or wood powders are used as the pore forming agent, it
is necessary to add a large amount of water in order to make the body for ceramics
attain a prescribed hardness in the kneading process, so that the efficiency in the
drying process becomes poor; and in the firing process the starches and wood powders
rapidly burn between 200 and 400°C to release a large amount of heat, so that it is
difficult to prevent the firing cracking. As above, in the prior art, it has been
extremely difficult to increase the porosity beyond a prescribed value.
Disclosure of the Invention
[0009] As a result of a diligent investigation performed in view of the above described
problems in the prior art, the present inventors reached the present invention based
on the findings that a very low pressure loss and a high collection efficiency can
be attained when the porosity of the honeycomb structure body is increased to a prescribed
value or more, and the total sum of the areas of the pores exposed on the partition
wall surfaces is made to a prescribed value or more, with paying attention to the
importance of the pore area on the partition wall surfaces with which surfaces the
exhaust gas actually comes into contact and through which surfaces the exhaust gas
passes.
[0010] In other words, according to the present invention, there is provided a honeycomb
ceramics structure body having chemical composition of 42 to 56 wt% of SiO
2, 30 to 45 wt% of Al
2O
3 and 12 to 16 wt% of MgO, and the crystalline phase mainly composed of cordierite,
characterized in that said honeycomb ceramics structure body has a porosity of 55
to 65%, an average pore size of 15 to 30 µm; and the total area of the pores exposed
on the surfaces of the partition walls constituting the honeycomb ceramics structure
body being 35% or more of the total area of the partition wall surfaces.
[0011] In the honeycomb ceramics structure body of the present invention, it is preferable
that the total area of the pores exposed on the partition wall surfaces is 40% or
more of the total area of the partition wall surfaces, and the average pore size is
from 15 to 25 µm. Furthermore, it is preferable that the partition wall thickness
is 300 µm or less. In addition, the permeability preferably is from 1.5 to 6 µm
2. It is also preferable that the coefficient of thermal expansion of the honeycomb
ceramics structure body of the present invention between 40 and 800°C is 0.5 × 10
-6/°C or less.
[0012] The honeycomb ceramics structure body of the present invention can be suitably used
as a diesel particulate filter (DPF) collecting the particulates discharged form a
diesel engine.
[0013] In addition, according to the present invention, there is provided a method for producing
a honeycomb ceramics structure body having chemical composition of 42 to 56 wt% of
SiO
2, 30 to 45 wt% of Al
2O
3 and 12 to 16 wt% of MgO, the crystalline phase mainly composed of cordierite, a porosity
of 55 to 65%, an average pore size of 15 to 30 µm; and the total area of the pores
exposed on the surfaces of the partition walls constituting the honeycomb ceramics
structure body being 35% or more of the total area of the partition wall surfaces,
characterized in that 15 to 25 wt% of graphite and 5 to 15 wt% of a synthetic resin
are added as a pore forming agent to a cordierite-forming raw material, the resultant
is kneaded and molded into a honeycomb shape, and thus formed body is dried and fired
to produce a honeycomb ceramics structure body.
[0014] In the above description, the synthetic resin is preferably any one of poly(ethylene
terephthalate) (PET), poly(methyl methacrylate) (PMMA), and phenolic resin, or a combination
thereof, and the average particle size of the raw material talc in the cordierite-forming
raw material is preferably 50 µm or less and the average particle size of the raw
material silica is 60 µm or less.
Brief Description of the Drawings
[0015]
Figure 1 shows a SEM photograph of the rib section in the honeycomb ceramics structure
body of Example 1.
Figure 2 shows a SEM photograph of the surface of the partition wall (membrane surface)
in the honeycomb ceramics structure body of Example 1.
Figure 3 shows a SEM photograph of the rib section in the honeycomb ceramics structure
body of Example 5.
Figure 4 shows a SEM photograph of the surface of the partition wall (membrane surface)
in the honeycomb ceramics structure body of Example 5.
Figure 5 is a graph showing the relationships between percent weight reductions (TG)
and heat flows (DTA) of the samples from Comparative Example 7 and Example 7.
Figure 6 is a graph showing the relationship between the soot deposition time and
the pressure loss.
Best Mode for Carrying out the Invention
[0016] The honeycomb ceramics structure body of the present invention has the chemical composition
42 to 56 wt% of SiO
2, 30 to 45 wt% of Al
2O
3, and 12 to 16 wt% of MgO, the crystalline phase mainly composed of cordierite, the
porosity of 55 to 65%, the average pore size of 15 to 30 µm, and the total area of
the pores exposed on the surface of the partition wall constituting the honeycomb
ceramics structure body being 35% or more of the total area of the partition wall
surface.
[0017] In the honeycomb ceramics structure body of the present invention, the porosity ranges
from 55 to 65%. With the porosity below than 55%, the pressure loss of the exhaust
gas is unpreferably increased, while with the porosity exceeding 65%, the mechanical
strength of the honeycomb structure body is so remarkably degraded that the honeycomb
structure body cannot be endurable to the actual use.
[0018] In addition, in this honeycomb ceramics structure body, the average pore size is
15 to 30 µm, and preferably from 15 to 25 µm. With the average pore size below than
15 µm, the collection efficiency is increased, but the pressure loss unpreferably
becomes high. On the other hand, with the average pore size exceeding 30 µm, the pressure
loss is satisfactorily low, but there occurs an increase in the probability that the
particulates in the exhaust gas are not collected since they pass through the larger
pores. In particular, when the wall thickness of the partition wall of the honeycomb
ceramics structure body is 300 µm or less, the degradation of the collection efficiency
becomes remarkable. In addition, when the average pore size exceeds 30 µm and the
porosity is below 55%, the initial pressure loss is low, but with increasing time
of use the pressure loss tends to increase sharply. It is considered that the particulates
in the exhaust gas tend to be deposited in the interior of the partition wall by passing
through the large pores, and there is caused an increase in the possibility that the
deposited particulates remain unburned when renewed by combustion. In addition, it
is also considered that even with a continuous renewal type honeycomb body supporting
an oxidation catalyst on the partition wall surface, the particulates similarly remain
unburned and are deposited in the interior of the partition wall to increase the pressure
loss. Accordingly, it is more preferable that the average pore size falls within the
range from 15 to 25 µm.
[0019] In addition, in the present invention, the total area of the pores exposed on the
surface of the partition wall constituting the honeycomb ceramics structure body is
35% or more of the total area of the partition wall surface. In such a manner, by
making the total area of the pores exposed on the surface of the partition wall be
a prescribed value or more, a high collection efficiency can be attained with a very
low pressure loss in relation to the exhaust gas. Incidentally, it is preferred that
the total area of the pores exposed on the partition wall surface is 40% or more of
the total area of the partition wall surface, and that it is 60% or less.
[0020] In addition, in the honeycomb ceramics structure body of the present invention, the
permeability can be made to range from 1.5 to 6 µm
2. A honeycomb structure body having the permeability of this range can attain a high
collection efficiency with a low pressure loss in relation to the exhaust gas.
[0021] Here, the permeability in the present specification means a numerical value obtained
by the following formula 1:

[0022] "In the above formula, C denotes the permeability (µm
2), F the gas flow rate (cm
3/s), T the sample thickness (cm), V the gas viscosity (dynes·s/cm
2), D the sample diameter (cm), and P the gas pressure (PSI). In addition, as for the
numerical values in the above formula, the following relations hold: 13.839 (PSI)
= 1 (atm), and 68947.6 (dynes/cm
2) = 1 (PSI)."
[0023] In the honeycomb ceramics structure body of the present invention, the coefficient
of thermal expansion between 40 and 800°C can be made to 0.5 × 10
-6/°C or less. With such a coefficient of thermal expansion, an excellent thermal shock
resistance is exhibited, so that the honeycomb structure body will be hardly damaged
even when sharp temperature changes repeatedly occur.
[0024] In addition, since as described above the honeycomb ceramics structure body of the
present invention is high in collection efficiency, it can be suitably applied to
a thin-wall honeycomb structure body having such a partition wall thickness of 300
µm or less.
[0025] Accordingly, the honeycomb ceramics structure body of the present invention having
the above described constitution can be very preferably applied as a diesel particulate
filter (DPF) collecting the particulates discharged from a diesel engine.
[0026] Then, description will be made on the method for producing a honeycomb ceramics structure
body according to the present invention.
[0027] The honeycomb ceramics structure body according to the present invention can be produced
through the following sequence of processes: firstly, a cordierite-forming raw material
is prepared in which talc, kaoline, calcined kaoline, alumina, aluminum hydroxide,
and silica are blended in such prescribed ratios that the chemical composition is
within a range capable of containing 42 to 56 wt% of SiO
2, 30 to 45 wt% of Al
2O
3, and 12 to 16 wt% of MgO; 15 to 25 wt% of graphite and 5 to 15 wt% of a synthetic
resin such as PET, PMMA, and phenolic resin are added as a pore forming agent, and
methylcelluloses and a surfactant are added in the prescribed amounts to the raw material,
and subsequently appropriate amount of water is added; the resultant mixture is kneaded
to form a body for ceramics. Then, the body for ceramics is subjected to vacuum degassing,
subsequently extruded into a honeycomb structure, dried by dielectric drying, microwave
drying, or hot air drying, and subsequently fired within a temperature of 1400 to
1435°C as a highest temperature, to produce the honeycomb ceramic structure body of
the present invention.
[0028] In addition, the staggered pattern clogging of the end surfaces in the honeycomb
ceramics structure body is performed after the drying process, or after the firing
process where the honeycomb structure body is fired again.
[0029] The production method of the present invention is characterized in that, to the cordierite-forming
raw material, 15 to 25 wt% of graphite is added as a pore forming agent, and simultaneously
5 to 15 wt% of a synthetic resin such as PET, PMMA, or phenolic resin all of which
is low in heat flow during combustion are added. By doing so, it has become possible
to produce, inexpensively and in a large scale, a cordierite honeycomb structure body
having a porosity of 55% or more.
[0030] With the addition of graphite exceeding 25 wt% in relation to the cordierite-forming
raw material, it is difficult to perform a uniform drying by the dielectric drying
or the microwave drying, and simultaneously it is required that in the firing process,
the combustion time within the range from 800 to 1000°C, in which range graphite is
combusted, is required to be made longer so as to suppress the rapid combustion of
the graphite. When the temperature rising rate in the combustion range of graphite
is too large, graphite is combusted rapidly to result in a wide temperature distribution
in the honeycomb structure body, involving a risk of generating cracks. In addition,
when graphite remains unburned, it affects adversely the cordierite-forming reaction
to be performed in a high temperature of 1200°C or above, involving a risk of increasing
the thermal expansion. Thus, in view of the industrial large-scale production, the
addition amount of graphite is required to be 25 wt% or less, and is more preferably
20 wt% or less. The lower limit for the addition amount of graphite is required to
be 15 wt% or more in view of the pore forming property and heat flow.
[0031] In the present invention, by adding a prescribed amount of a synthetic resin relatively
low in heat flow during combustion to the graphite, it becomes possible to produce
a honeycomb structure body having such a large porosity as is 55% or more.
[0032] In addition, in order to enlarge the total area of the pores exposed on the surface
of the partition wall of a honeycomb structure body, as in the honeycomb ceramics
structure body of the present invention, it is necessary to increase the porosity
and simultaneously it is also necessary to control the pores formed by talc and silica
in the process of the cordierite-forming reaction. When the talc raw material or the
silica raw material is made to be coarse particles, it is possible to make the average
pore size be larger, but the formed pores do not necessarily appear on the partition
wall surfaces, and it results in the formation of coarse pores in the interior of
the partition walls. This is because the coarse particles tend to gather together,
during extrusion, in the central part of the partition wall.
[0033] Thus, in the present invention, it becomes possible to form pores effectively on
the surface of the partition walls by controlling the average particle size of the
talc raw material to 50 µm or less and the average particle size of the silica raw
material to 60 µm or less, both material being important for forming pores; consequently,
the ratio of the total area of the pores exposed on the surfaces of the partition
walls in a honeycomb structure body to the total area of the partition wall surfaces
can be made to be 35% or more. Moreover, it is more preferable that the average particle
size of the talc raw material is within the range of from 20 to 50 µm and the average
particle size of the silica raw material is within the range of from 20 to 60 µm.
[0034] Description will be made below on the present invention on the basis of the specific
Examples, but the present invention is not limited to these Examples.
(Examples 1 to 11 and Comparative Examples 1 to 16)
[0035] The cordierite-forming raw materials and pore forming agents shown in Table 1 were
blended in the respective content ratios shown in Table 2; to the mixtures thus obtained
methylcellulose and hydroxypropoxyl methylcellulose were added by 2 wt%, respectively,
then a fatty acid soap was added as a surfactant by 0.5 wt%, and further an appropriate
amount of water was added, to form respective puddles. Then, using these puddles,
as shown in Table 3, a series of honeycomb structure bodies of φ150 mm × 150 mm (length)
were extruded, where the cell structure was such that either the wall thickness was
300 µm and the number of the cells 31 cells/cm
2, or the wall thickness was 430 µm and the number of the cells 16 cells/cm
2. The respective extruded bodies were subjected to the dielectric drying and hot air
drying to remove water. Then, the formed body was fired under the conditions that
the highest temperature was 1415°C and the retention time at the highest temperature
was 8 hours; both end faces were clogged alternately in a staggered pattern with a
slurry-like cordierite-forming raw material, then the respective formed bodies were
again fired with the highest temperature of 1420°C, and thus respective honeycomb
ceramics structure bodies as an evaluation sample were produced.
[0037] Here, the measurements of the average pore size, porosity, ratio of the total area
of the pores exposed on the partition wall surfaces to the total area of the partition
wall surfaces (area ratio), permeability, coefficients of thermal expansion between
40 and 800°C (CTE), pressure loss, and collection efficiency of a honeycomb ceramics
structure body were performed as follows.
[0038] The average pore size and the porosity were obtained from the pore distribution measured
by the mercury intrusion method. The porosity was calculated from the total pore volume.
[0039] Area ratio: the area ratio of the pores exposed on the partition wall surfaces was
obtained by analyzing a photograph of the partition wall surfaces obtained by the
SEM observation using an image analysis apparatus.
[0040] CTE: the measurement was made with the differential measurement method using a quartz
standard specimen.
[0041] Permeability: a portion of the partition wall was cut out from a honeycomb ceramics
structure body, and was so processed that the concavities and convexities were removed
to prepare a test sample; the sample was so placed between a pair of members of the
sample holder of φ20 mm that no gas leaks, with one member in contact with the top
face of the test sample and the other in contact with the bottom face of the test
sample, and then a gas was made to flow into the sample holder at a specified gas
pressure; and the permeability was obtained from the gas amount which passed through
the test sample on the basis of the following formula 1:

[0042] "In the above formula, C denotes the permeability (µm
2), F the gas flow rate (cm
3/s), T the sample thickness (cm), V the gas viscosity (dynes·s/cm
2), D the sample diameter (cm), and P the gas pressure (PSI). In addition, as for the
numerical values in the above formula, the following relations hold: 13.839 (PSI)
= 1 (atm), and 68947.6 (dynes/cm
2) = 1 (PSI)."
[0043] Pressure loss: soot was generated using a light oil gas burner, and a DPF was arranged
at a position downstream of the burner; the combustion gas containing the soot was
made to flow into the DPF at the gas flow rate of 2.4 Nm
3/min and the temperature of about 150°C; and the pressure loss was obtained from the
time variation of the pressure difference between before and after the DPF measured
while the soot was being deposited in the DPF.
[0044] Collection efficiency: soot was generated using a light oil gas burner, and a DPF
was arranged at a position downstream of the burner; the combustion gas containing
the soot was made to flow into the DPF at the gas flow rate of 2.4 Nm
3/min and the temperature of about 150°C; and the collection efficiency was obtained
from the weight ratio between the soot weights in the definite fractional gas flows
respectively branched from the gas flow respectively at some positions upstream and
downstream of the DPF.
(Discussion)
[0045] Figure 1 shows a SEM photograph of the rib section of the honeycomb ceramics structure
body of Example 1, and Figure 2 shows a SEM photograph of the partition wall surface
(membrane surface) of the honeycomb ceramics structure body of Example 1.
[0046] In addition, Figure 3 shows a SEM photograph of the rib section of the honeycomb
ceramics structure body of Comparative Example 5, and Figure 4 shows a SEM photograph
of the partition wall surface (membrane surface) of the honeycomb ceramics structure
body of Comparative Example 5.
[0047] In the two photographs of Figures 2 and 4, the white looking areas (the yellow looking
areas in the photographs substituted for drawings) are the pores exposed on the partition
wall surface (surface pores). The high area ratio of the surface pores leads to the
decrease of the initial pressure loss.
[0048] Figures 3 and 4 show the fine structure of the honeycomb structure body of Comparative
Example 5.
[0049] From the photographs of Figures 3 and 4, it can be seen that in the rib section of
Comparative Example 5, very large pores gather together near the central part of the
rib, owing to the use of the silica raw material comprising coarse particles having
the average particle size of 75 µm. Large pores are known to be formed by use of coarse
particle raw material of talc or silica; however, the coarse raw material particles
gather together near the central part of the ribs when the honeycomb structure is
formed by extrusion, and hence the large pores are formed only in the central part
of the ribs. In the photograph of the membrane surface of Comparative Example 5, the
total area of the pores exposed on the partition wall surfaces was only 20%. Comparative
Example 5 is not so high in pressure loss, but the collection efficiency is as poor
as 80% owing to the effect ascribable to the large pores.
[0050] In Example 1 as shown in Figures 1 and 2, a synthetic resin of PET was used as a
pore forming agent together with graphite in order to increase the porosity. As a
result, the porosity became as high as 63%. It was also confirmed that the use of
a synthetic resin had the effects of increasing the porosity of the honeycomb structure
body, and simultaneously increasing the surface pores as can be seen from the appearance
of the top end face and that of the bottom end face of the rib section in Figure 1.
When the photograph of Figure 2 was subjected to the image analysis, the area ratio
of the surface pores in Example 1 was found to be as high as 45%, and consequently,
as shown in Table 3, the permeability was 4.8 µm
2, the initial pressure loss was suppressed to a very low level of 65 mmHg, and the
collection efficiency reached a level as high as 95%.
[0051] Conventionally, starch and the like have been used as the pore forming agents substituting
for graphite, but when used in a large amount, there occurs a problem that "fissure"
is generated in the drying process, firing process, and the like. Figure 5 shows the
relationship between the percent weight reduction (TG) and heat flow (DTA) in the
dough (Comparative Example 7: batch 6-2) containing starch (cornstarch) in 10 wt%
and graphite in 20 wt%, and that in the dough (Example 7: batch 12) containing a synthetic
resin (PET) in 10 wt% and graphite in 20 wt%.
[0052] As can be seen from Figure 5, when starch was used as a pore forming agent, the starch
was thermally decomposed around the temperature range from 300°C to 350°C to sharply
release heat (see the dotted line for DTA), and the resulting thermal stress generated
the fissure in the firing process. However, there was found an advantage that when
PET, PMMA, phenolic resin, crosslinked polystyrene, or the like was used as a pore
forming agent, the heat flow in the concerned temperature range was suppressed to
such a low level (see the solid line for DTA) that the fissure was very scarcely generated
in the firing process.
[0053] Figure 6 is a graph showing the relationships between the soot deposition time and
the pressure loss.
[0054] In Figure 6, the solid line shows the result for the honeycomb structure body of
Example 1, and the broken line in Figure 4 shows the result for the honeycomb structure
body of Comparative Example 5.
[0055] The conditions for the soot deposition were that a light oil gas burner was used
to generate a gas having a temperature of about 150°C, and the gas thus generated
was made to flow into the DPFs made of the honeycomb structure bodies of Example 1
and Comparative Example 5 at a gas flow rate set of 2.4 Nm
3/min.
[0056] As can be seen from the results shown in Figure 6, for the honeycomb structure body
of Example 1 in which the porosity was 63% and the area ratio was as large as 45%,
the pressure loss rise did not become large even after a prescribed elapsed time,
whereas for the honeycomb structure body of Comparative Example 5 in which the porosity
was 50% and the area ratio was as small as 20%, the pressure loss rise became large
with elapsing time.
Industrial Applicability
[0057] As described above, according to the present invention, it is possible to provide
a honeycomb ceramics structure body which can attain a low pressure loss and a high
collection efficiency, and a method for producing the same.
1. A honeycomb ceramics structure body having chemical composition of 42 to 56 wt% of
SiO2, 30 to 45 wt% of Al2O3 and 12 to 16 wt% of MgO, and the crystalline phase mainly composed of cordierite,
characterized in that said honeycomb ceramics structure body has a porosity of 55 to 65%, an average pore
size of 15 to 30 µm; and the total area of the pores exposed on the surfaces of the
partition walls constituting the honeycomb ceramics structure body being 35% or more
of the total area of the partition wall surfaces.
2. The honeycomb ceramics structure body according to claim 1 in which the total area
of the pores exposed on the surface of the partition wall is 40% or more of the total
area of said partition wall surface.
3. The honeycomb ceramics structure body according to claim 1 or 2 in which the average
pore size is from 15 to 25 µm.
4. The honeycomb ceramics structure body according to any one of claims 1 to 3 in which
the thickness of the partition wall is 300 µm or less.
5. The honeycomb ceramics structure body according to any one of claims 1 to 4 in which
the permeability is from 1.5 to 6 µm2.
6. The honeycomb ceramics structure body according to any one of claims 1 to 5 in which
the coefficient of thermal expansion between 40 and 800°C is 0.5 × 10-6/°C or less.
7. The honeycomb ceramics structure body according to any one of claims 1 to 6, which
can be used as a diesel particulate filter.
8. A honeycomb ceramics structure body having chemical composition of 42 to 56 wt% of
SiO2, 30 to 45 wt% of Al2O3 and 12 to 16 wt% of MgO, the crystalline phase mainly composed of cordierite, a porosity
of 55 to 65%, an average pore size of 15 to 30 µm; and the total area of the pores
exposed on the surfaces of the partition walls constituting the honeycomb ceramics
structure body being 35% or more of the total area of the partition wall surfaces,
characterized in that 15 to 25 wt% of graphite and 5 to 15 wt% of a synthetic resin are added as a pore
forming agent to a cordierite-forming raw material, a resultant is kneaded and molded
into a honeycomb shape, and thus formed body is dried and fired to produce a honeycomb
ceramics structure body.
9. The method for producing a honeycomb ceramics structure body according to claim 8
wherein the synthetic resin is any one of poly(ethylene terephthalate) (PET), poly(methyl
methacrylate) (PMMA), crosslinked polystyrene, and phenolic resin, or a combination
thereof.
10. The method for producing a honeycomb ceramics structure body according to claim 8
or 9, wherein the average particle size of the raw material talc is 50 µm or less,
and the average particle size of the raw material silica is 60 µm or less, in the
cordierite-forming raw material.